Physico-chemical properties and treatment of scale formation in dust scrubber discharge lines at a PGM Smelter in South Africa

Abstract

Several technologies in the field of flue-gas desulphurization (FGD) have been created to address the issue of sulphur dioxide (SO2) emission from gas streams. Among these, wet scrubbing, particularly the use of lime/ limestone (Ca(OH)2/ CaCO3) scrubbing, stands out as the primary method for reducing SO2 emissions from power plants. These methods are simple and cost-effective, making them suitable for various industrial facilities that emit SO2, including refineries and smelters. In Ca(OH)2 or CaCO3 scrubbing systems, calcium (Ca) compounds are introduced in the form of slurries into the scrubber liquid. However, this process leads to the undesired creation of solid Ca salts. Consequently, the solubility of Ca salts in the slurry restricts the efficiency of wet scrubbing techniques containing Ca. If the ion concentration in the water exceeds the solubility limit of Ca salts like calcium sulphate (CaSO4), it can result in the development of supersaturated CaSO4, which may lead to scale accumulation or deposition in the scrubber. This scaling issue, in turn, requires frequent plant shutdowns to open and remove scale from pipelines. The aim of this research is to propose a process for the prevention of hard water scale or its removal in FGD systems, particularly scrubbers commonly utilized in PGM smelters. To accomplish this goal, analyses were conducted to both physically and chemically characterize the scale or deposit, as well as all the feed materials within and around the variable throat scrubber (VTS) system at a local PGM smelter. By leveraging the physical and chemical properties of these materials, this study explores the application of traditional chemical "water softening" techniques like ion exchange and precipitation, as well as an emerging physical method known as magnetic water treatment (MWT), to combat scale formation in scrubbers. The water samples obtained from the Sibanye-Stillwater scrubbing circuit were characterized by extremely high levels of Ca and magnesium (Mg) hardness (1000—8000 mg/L CaCO3) and high levels of total dissolved solids (TDS) (3000—9000 mg/L). Both a strong acid cation exchange resin (SAC) and weak acid cation exchange resin (WAC) were employed in the treatment process. The WAC resin, commonly used for high TDS solutions, displayed better removal of Ca and Mg compared to SAC, effectively bringing the total hardness levels down to 120—180 mg/L CaCO3. Chemical precipitation using a lime and soda ash pre-treatment step prior to cation exchange resulted in residual hardness levels of 0—120 mg/L as CaCO3. Physico-chemical properties and treatment of scale formation in dust-scrubber discharge lines at a PGM smelter in South Africa Thandiwe Fungene VI The use of MWT remains a topic of debate as a non-chemical option for water softening due to concerns about its scientific validity. This research aims to investigate the potential of a magnetic field to reduce hard water scaling. Several factors, including pH, dissolved oxygen (DO), electrical conductivity (EC), calcium ion concentration ([Ca2+]), and scaling potential, were compared between treated and untreated water. The treated water displayed notable changes in these factors. Most significantly, the precipitation in treated water indicated a shift from calcite to aragonite formation, essentially inhibiting the overall scaling potential. These findings are substantiated by a mechanistic theory based on a comprehensive review of successful applications in the existing literature. This study holds significance in questioning the sustainability of chemical-based water treatment methods and explores the feasibility of non-chemical alternatives.